avcodec/mlp_parser: Check if synccode is within buffer

Fixes: undefined shift
Fixes: 9216/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEHD_fuzzer-6281404575907840

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mlp_parser.c

@@ -320,7 +320,7 @@ static int mlp_parse(AVCodecParserContext *s,
         mp->bytes_left = 0;
     }
 
-    sync_present = (AV_RB32(buf + 4) & 0xfffffffe) == 0xf8726fba;
+    sync_present = buf_size >= 8 && (AV_RB32(buf + 4) & 0xfffffffe) == 0xf8726fba;
 
     if (!sync_present) {
         /* The first nibble of a frame is a parity check of the 4-byte